Social norms affect environmental quality. But what exactly is a social norm? Environmental economists studying the topic draw on diverse scholarly traditions and may not have the same phenomenon in mind when using the concept. For example, social norms may refer to common, but not necessarily socially approved, behaviors; to internalized ethical rules; or to one of several equilibria in a coordination game. I first discuss some of the definitions used in the environmental economics literature. Then, I outline a simple framework for analysis of voluntary contributions to public goods. Using this framework, I illustrate differences and similarities between altruism, moral norms, and social norms and discuss implications for environmental policies. In particular, when a social norm represents one of several stable equilibria, policy can potentially invoke abrupt and dramatic behavioral changes.